1. Field of the Invention
The invention relates to projection displays and more particularly an improved method of homogenizing and formatting the light from a light source to produce higher uniformity and efficiency in the projected range.
2. Description of Related Art
Illumination systems used for image projectors are designed to generate a spatially uniform plane which can be used to illuminate an imaging device film or other media. The reflected or transmitted light from the imaging device is then projected onto a screen for viewing. The brightness and spatial brightness uniformity should be within certain limits for each particular application to be considered acceptable to the viewers.
Image projectors including film movie projectors, slide projectors, electronic liquid crystal and micro-electro-mechanical (mem) projectors, microfilm and overhead projectors all require a high degree of spatial light uniformity in the image to produce a pleasing image. This has always been a challenge for projection system designs due to the fact that the light sources available for these systems all have very disorganized light output and therefore require complex optical systems to organize the light. Additionally, high degrees of magnification in short distances (which often occur in these optical systems) cause a problem which is well known in the optical fieldxe2x80x94the cosine4 roll off of power in the image as you move radially away from the center of the image. This effect is most predominant at the corners of the image. Another problem is that light sources tend to produce round or elliptical gaussian beam profiles, while most images are rectangular in format. Typically, the light beam is spatially truncated (i.e., the portions of the beam which fall outside a rectangular profile that corresponds to the image are blocked). This leads to another problem, which is maximizing the brightness of the illuminationxe2x80x94when the light is truncated to change its geometry, the truncated light is obviously wasted.
Many optical methods have been used in the prior art to try to minimize the variations in uniformity which are due to the particular characteristics of the available light sources as well as to maximize the brightness of the illumination.
The optical method used depends somewhat on the light source used. Many different types of light sources are in common use today. Some types are electric filament, and arc lamps including metal halide arc, low and high pressure mercury arc, xenon arc, carbon arc, as well as solid state Light Emitting Diode (LED) sources, and Lasers. Not all of these light sources, however, are suitable for displays using prior art technologies.
Two of the most common types of light sources in use in commercial applications are metal halide arc lamps and high pressure mercury arc lamps.
These arc lamps are usually configured in an optical illumination system which employs an elliptical or parabolic reflector to gather and direct the light to a focal point or collimated beam respectively, as shown in FIG. 1. Both of these types of systems produce highly non-uniform beams. Some systems use reflective tunnels or light pipes through which the source light is channeled in order to create a scrambled, hence more spatially uniform bundle of light rays as shown in FIG. 2.
Lenslet arrays are also sometimes used to increase the uniformity of the light. Some versions of these lenslets are described in U.S. Pat. Nos. 5,098,184 and 5,418,583. The lenslet arrays function essentially in the following manner. Two lenslet arrays are separated by a distance equal to the focal length of the individual elements. The elements of the first array form an image of the source in the aperture of the elements of the second array. In the case of a laser, the source image is a diffraction pattern. The elements of the second array then form an image of the aperture of the elements of the first array on the illumination plane. The aperture is chosen to match the aspect ratio of the device (film gate, or LCD) to be illuminated. A field lens in close proximity to the second array focuses the chief rays of each element to the center of the illumination plane so that the subsets of the beam sampled by all elements of the arrays are superimposed at the illumination plane and an averaging process thus occurs that causes the illumination plane to have more uniform irradiance. A second field lens is often required at the illumination plane to ensure that the light is telecentric as most often required by projection imaging optics.
In this manner a beam with non-uniform irradiance may be sampled by arrays composed of many elements and converted to a uniform beam with a different geometry (generally rectangular).
The lenslet array optical system which is used in an illumination system has design characteristics that must be adjusted to ensure that the illumination and imaging systems are compatible. If they are not, then light is wasted. For example, the geometry of the illumination should be the same as the geometry of the imager. The numerical aperture of the illumination system should also be compatible with the imaging system. The ratio of the footprint of light incident on the first array to the distance to the illumination plane determines the numerical aperture of the illumination light. Thus the focal length of the array elements and the field lens focal lengths are adjusted to ensure that the illumination numerical aperture matches the imaging numerical aperture.
At first blush, laser light appears to have enormous potential for being the illumination source in projection display systems. The light is well behaved and organized (ie: it is collimated), it is chromatically pure, and with a minimum of three wavelengths (Red, Green, and Blue) a high color space or gamut can be created, and high power low cost lasers are becoming available. There are, however, several problems with laser-based illumination systems.
First, the coherency of laser light leads to speckle, which is a fine-grained non-uniformity. The speckling effect is increased with the use of so-called holographic diffusers as proposed in this invention. The net effect is often a high frequency mottling effect sometimes called worminess. Another problem is that the laser light is collimated and, as such, it is difficult to create a cone or numerical aperture which will allow an image to be projected onto a screen, as with a projector. Yet another problem is that the laser light typically has a gaussian intensity profile and it may have a wide range of diameters, depending upon the particular laser source which is used. This can, and often does, lead to a non-uniform light distribution on the final screen or projected image surface.
Another problem is that currently available lasers typically do not have enough power to provide sufficient illumination in some display devices. Further, using prior art methods, it is difficult to combine the beams of multiple lasers to obtain sufficient illumination for this purpose.
Another problem with the use of laser light as a display illumination device is that the beam generated by a laser may be astigmatic in its divergence. In other words, the divergence in the beamxe2x80x9ds cross section may be greater in one axis than another. This causes additional processing problems compared to a circularly symmetric diffraction limited beam.
Yet another problem with the use of laser light in a display illumination device is that, if laser light is diffracted in an optical system, a certain amount of light passes through the diffracting device without being diffracted. This effect is referred to as zero-order light leak. Zero-order light leak may prevent the resulting diffraction pattern from conforming to a well-defined, desired function. Another problem with using laser light sources for illumination is that they are monochromatic. Since it is desirable to have a source of white light, it may be necessary to combine laser light beams of several different wavelengths (e.g., red, green and blue.) This may be difficult because many optical systems and components are wavelength-dependent and may therefore require color correction to provide even illumination.
Another problem with the use of laser light in display systems is that a large physical volume is normally required. The space requirements of these systems results in part from the separate processing of the laser illumination light in a first optical system and the subsequent processing of the image information in a second optical system so that it can be displayed for viewing.
Yet another problem with the use of laser light in a display illumination device is that optical processors for formatting the illumination image from the laser source are configured to provide a single fixed illumination aspect ratio format. To obtain a different aspect ratio format for use in the display, the illumination source is typically masked, so a portion of the light is lost and significant system efficiency is lost. In order to utilize all of the light generated by the laser source, it may therefore be necessary to use an entirely different optical processor.
One or more of the problems outlined above may be solved by the various embodiments of the invention. The present invention performs a similar function as a lenslet array optical system, but does so more effectively, with fewer and lower cost components, and with improved design flexibility. The present techniques may be applied to many types of illumination sources such as arc lamps and LEDxe2x80x9ds in addition to lasers.
Broadly speaking, the invention comprises a system and method for converting a laser beam having a non-uniform profile into a source of illumination which has uniform power density. The generated illumination image may be used for a variety of purposes. For example, the image may be a uniformly intense rectangle suitable for use in a display device, or it may be a round dot suitable for transmitting the light into an optical fiber. The present invention can be used to conserve the power generated by the laser source and direct substantially all of the power into the desired illumination region. Laser speckle artifacts can also be reduced or eliminated at the same time. The choice of design of the elements in the system allows for precise control of the illumination pattern and the particular telecentric cone angle patterns exiting the illumination pattern. While the preferred embodiment uses a laser source, the system is capable of utilizing a wide variety of light source devices, including all arc lamps and LED sources.
The operation of a system in accordance with one embodiment of the invention is as follows. A block diagram of the system is shown in FIG. 4. A beam of light is first generated by the laser light source. The light beam is expanded or sized to illuminate a controlled angle diffuser. The expanded beam remains collimated.
The expanded beam is passed through a controlled angle diffuser (e.g., hologram, bulk scatterer, etc.) to diffract or direct the light in a predetermined pattern. (Crossed lenticular arrays, or lenslet arrays can also be used.) The controlled angle diffuser can be designed to emit light angularly in any geometry (such as rectangular to match a display device aspect ratio). The angular emission of a holographic diffuser is similar to the aperture geometry of the lens array system described above. It should be noted, however, that in the prior art it takes two optical elements with an intervening space to produce an effect which is performed by a single optical element (the holographic diffuser) in the present system.
A first field lens is positioned following the holographic diffuser. This first field lens focuses and spatially overlays the diffracted light onto a single rectangular plane which lies at a distance from the lens equivalent to its focal length. A second field lens is used at this illumination plane to correct for the degree of telecentricity desired in the system. In some cases, over-correction or under-correction may be desired. This image is then used as the illumination source for a display. Both field lenses function identically to field lenses in lens array systems, but at significantly lower cost.
The present systems and methods may provide a number of advantages over prior art. For instance, the level of light efficiency may be substantially increased over the prior art. Further, the problems often encountered in coherent optical systems relating to speckle and image worminess (high frequency intensity variation) may be reduced or eliminated. Another advantage is that the illumination provided in this manner is uniform and can be spatially formatted to match the display device being illuminated (rather than providing illumination with the gaussian intensity falloff which is common in prior art designs).
An alternative to the holographic diffuser is a crossed lenticular array as shown in FIG. 5A. The crossed lenticular array performs the same optical function as the hologram for a rectangular emission profile, but at a lower spatial sampling rate. The lens profiles in the lenticular can be aspheric to compensate for uniformity issues as described above. The crossed lenticulars can be combined into one element as shown in FIG. 5B. An additional configuration is to integrate the crossed lenticular function into a single element lenslet array as shown in FIG. 5C. While the lenslet arrays reduce the beam sampling rate and thereby slightly reduce the resulting image uniformity, they are significantly more achromatic than holographic diffusers and can therefore be used with polychromatic light sources. This embodiment also provides a significant advantage over the prior art in that it does not require the intervening space and volume between the prior art lenslet arrays and thereby allows for construction of more compact systems.